Temperature affects the rate of rose shoot development. In this study heat units were used to quantify “physiological age”. The objective was to determine whether rose shoots require the same number of heat units to reach various stages regardless of climatic conditions. The dates of occurrence of “bud break” (BB), unfolding of each leaf, “visible flower bud” (VFB), and harvest (H) were observed for 126 shoots of 'Cara Mia' roses growing under 5 different temperature and light regimes. Average air temperature (T) and photosynthetic photon flux density (PPFD) levels were recorded hourly. Heat units, defined as the sum of the difference T-Tb (units: °C hr-1) where Tb is the base temperature, were found to be a suitable for tracking most phases of rose crop development. The duration of the phase from H to BB showed considerable variation and thus could not be predicted this way. The duration from BB to VFB or H could be predicted reasonably well to occur at 5900 ± 670 and 12300 ± 1000 °C hr-1 (Mean ± Std. dev.), respectively, assuming Tb =6 °C. The occurrence of unfolding of each leaf can be predicted similarly. PPFD integrals had no significant effect on any development rates.
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